1. Field of the Invention
The present invention relates to a data storage medium and, more particularly, to a portable data storage medium having a radio communication function.
2. Description of the Related Art
Radio cards and tags without any battery have recently been developed as portable data storage mediums having a radio communication function and have started to be put to practical use. These data storage mediums generally perform an intermittent operation of half duplex communications using ASK (amplitude-shift keying) modulation, and the amount of data which can be transmitted through one communication is small.
According to the conventional intermittent communication, a cycle of power reception, data reception and data transmission, for generating a power supply voltage in a radio card, is repeated every time a small amount of data of one bit is communicated. For example, the radio card is charged with a power carrier wave (for a predetermined period of time) to receive a command (indicative of reading or writing of data) or read/write data within a range of voltage used during the discharge. In other words, since a predetermined period of time for charging the radio card is always required before the reception of a command or the reading/writing of data, the reception or reading/writing cannot be performed continuously. Such intermittent communication causes a problem of making it difficult to continuously receive/transmit a large amount of data at high speed.